Electromagnetic clutch



March 23, 1954 A. c. MILLER ELECTROMAGNETIC CLUTCH Filed May 16, 1949 Q) b ANDREW RA/G MILLER Patented Mar. 23, 1954 ELE CTRQMAGNETIG CLUTCH Andrew Craig .Miller, Stoke, .Cnventry England assignor to Humber Limited, Stoke, England,-a British comp Coventry any Application May 16, 1949,=-Serial-No. -93,554 t Claimspriority, applicationGreat Britain,- May 21, 1948 1 Claim.. (Cl. 192103)'"- The. invention relates toelectro-magnetic clutches (e. g. for motor vehicles). of the kind compl udrivingfin driven re ateb e m mber and angeleotrical solenoid arranged pn-energiza; tion to efiect engagement of the clutchbybr-ing ing; the two members into frictional. or; magnetic ore. ansmiti eare at qn It asrb en ,f undiiha wiiha u dias f e e H ,e j r s nie cn mqted tfis- 1 ficult to obtain, on energization o f. the.- coil,; or solenoid, the degree of slip between the two members necessary to ensure smooth take-up of the drive when the torque to be transmitted on enasemen-tlisisubstantia y-J ss t enithe maximum torque;-ior--which the clutch-is designed, For instancein a vehicle having a high, power/weight ratio if the clutch isdesigned to be able to transmit the maximum torque thatthe engine can deliver it is found that-the clutch is fierce and difliculty. is experienced inv engaging .the clutch gently, asuis required to:.inchi thevvehicleiwhen. parking and driving in dense trafiic or to hold the vehicle on a very slight gradient. It is an object of the invention to provide an improved clutch of the kind described which overcomes or reduces this difficulty.

The invention provides an leleetro-magnetic clutch of the kind described in which the solenoid is constructedin two parts; theenergization of one of which parts may be controlled or effected separately from the othenthereby providing means by which the-torquetransmitting capacity of the clutch may be increased after the clutch has been engaged. Thisarrangement has the advantage that the torque handlingcapacity, of the clutch on engagement may be comparatively low enabling the slipbetween the members necessary for smooth engagement to take place and the capacitymay subsequently be. increased to enable the clutch to transmit with lit tle or no slip the full torque desired.

The clutch includes aiswitch in the supply circuit to one solenoid part and means for automatically closing the switch to energize the part whenbthe. speed of one of the clutch members (preferably the driving emember) exceeds a, predetermined value- Preferablythe two. solenoid nection in parallel.

In one construction of the clutch according to clutch to" provide a lim' ed torque transmitting capacity; and there is a centrifugal switch'for automatically connecting the other coll in' par allel with the first said coil when the speed of the driving member exceeds a predetermined value, so as to provide above that speed value an increased torque transmitting capacity. The two parts of the solenoid may have different-currentconsumptions and-have the same or-adifferent number of turns. Preferably-- the centrifugal switch-is arranged to be driven byyor in synchroa nism with, the driving friction memberonshait and employed to complete the-circuitof the-secto be energizedy-when the speed of the driving shaft exceeds the-said'predetermined value.-

Th invention may beappliecliori example to the magnetic-clutches described'inBritish patent specifications --Nos.- 594,906 and--597g193 A specific construction-oflan electr c-magnetic clutch incorproating the invention will now be described by way of example -and with reference to the accompanying drawings :in-whic-h':

Figure l is .asection. through the-clutch; and

Figure 2a view in the directionmfthe-arrow inFigure l shoWihgthe clutch with the cover' partlycut away.

Figure 3 is'a schematic-view showing thescir cult to the'coils.

The clutch forming the subject of thlslexam me is intended for use in transmitting the :drive gear box andvthence :to the road wheels-oi .a motor vehiclev One..of.-the clutchits face andrecessedinto-the-baseof the groove- The sides 8 of :the coils areso tapered are the magnet coils 6,: 7. groove outside the magnet that. the mouth ofthe groove-is wider thanthe base.

The other clutch-engaging member consists of an annular pressure flygwheel there a clutch surface IUa arranged tomake, engagement; with a frictional lining I2" on*one face ore -clutch plate Il'splined to the driven shaft IQ of the clutch, the driven shaft being in constant driving engagement with the gear box. The other face of the clutch plate l3, which also has a frictional lining I4 is arranged to make driving engagement with a ring |5 secured to the adjacent face of the fly-wheel.

Flexible strips of spring steel extend radially between the outer face of the pressure plate l0 and a peripheral flange 2| of non-magnetic material on the flywheel. The strips are attached by screws 22, 23 both to the pressure plate and to the fly-wheel flange and are arranged to transmit the torque between the two parts. The pressure plate I0 is received substantially wholly within the flange 2| on the fly-wheel which also surrounds the clutch plate l3. The strips 20 are slightly biassed to tend to disengage the clutch.

The solenoid consists of two separate coils 6, 1. Each coil is rectangular in section and the two coils have equal numbers of turns and are wound in the same direction. One coil 6, is wound around the outer periphery of the other, I, and the two coils fit within the annular groove in the face of the fly-wheel.

A casing surrounds the clutch and electrical connection is made to the coils by means of a brush 3| (or two diametrically opposed brushes) supported on the casing and making connection with a single insulated slip ring 32 secured to the pressure plate |0. The inner coil 1 is electrically connected directly between the slip ring 32 and the fly-wheel which is electrically connected to the frame of the vehicle via the driving shaft. Th outer coil 6 is connected to the sliprin through a centrifugal switch 35 mounted on the outer face of the pressure plate and the circuit is completed through the fly-wheel.

The connection from the slip-ring to the inner coil comprises a lead, not shown, from the slipring to a flexible strip 31 extending between the pressure plate and the flange 2| and a resilient connection a, as described in British specification No. 610,594 and similar to that shown at 40. The connection from the slip-ring to the outer coil comprises a lead 4| from the slip-ring to the centrifugal switch 35, a flexible strip 42 and the connection 40 with lead 43. The centrifugal switch comprises a plunger 45 movable under centrifugal action against a spring 46 to make contact with a button 41.

The method of operation is as follows: The desired gear is engaged and the clutch operating voltage is applied to the slip-ring by any convenient means (such as the variable output of an engine-driven generator 50 as described in our British specification No. 610,519) or from the car battery with or without a series resistance. The inner coil is thereby energized and the clutch is then able to transmit a limited torque, suflicient to creep the vehicle or to hold it on a slight gradient as the accelerator pedal is gently depressed or held. When the engine is speeded up the centrifugal switch operates when a predetermined speed is reached and closes the supply circuit to energize the outer coil. The clutch can then transmit th full engine torque.

It will be appreciated that by suitable design of the coils the clutch may be arranged so that when engaged by energization of only one of the coils, it may have any desired percentage of its full torque transmitting capacity. As an example of the application of this feature of the invention the following flgures give an indication of the required percentage of the available engine 4 torque (at 600 R. P. M.) which are required to maintain a typical modern high performance car having a rolling resistance of 38 lbs. per ton in motion on the level, at the speeds indicated and with the indicated gears engaged.

Actual Percent Gear Torque i g fi It will also be appreciated that by the employment of two coils a large range in torque transmitting capacity may be obtained without an inconveniently large range of current consumption, since the torque transmitting capacity of the clutch is proportional to the square of the ampere turns of the energized coil. Two similar coils will therefore, when both energized, give four times the torque transmitting capacity which is given by the energization of only one of the coils, but the current consumption will only be doubled.

For example, consider a clutch having an effective radius, r, of 6 inches and a friction material having a co-efiicient of friction, a, of 0.25. Then th axial pull, P, required to transmit a torque T is given by:

2p1 The flux density in kilogauss B, required to produce the axial pull P when the area of the magnetic gap is A sq. cms. is given by the formula:

In this example A=200 sq. cms. and therefore in order to transmit a torque of lbs. feet:

and therefore B=4.'73 kilogauss.

The ampere-turns, At, required to produce this flux density are given by:

Similarly in order to produce a torque transmitting capacity of 400 lbs. feet (1. e., four tim that referred to above) therefore 8:9.46 kilo auss and At=2 9460 .020=3'78.4 ampere turns-that is twice the ampere turns needed to produce 100 lbs. feet torque capacity.

I claim:

An electro-magnetic friction clutch for use with an internal combustion engine having a driving shaft which clutch comprises a rotatable flywheel for attachment to the engine shaft and having a groove in one side face concentric with the shaft, two annular coils secured within the groove at the bottom thereof, a rotatable driven shaft coaxial with the axis of rotation of the flywheel, a pressure plate arranged for rotation about the driven shaft and for limited movement towards and away from the said face of-the fill-- wheel, an annular armature secured to the pressure plate and received within the aforesaid groove for attraction by the coils when energized,

References Cited in the file of this patent UNITED STATES PATENTS Name Date Reist May 12, 1903 Number Number Number Name Date Schuster et al Aug. 18, 1903 Howell July 9, 1907 Libby Mar. 20, 1917 Brainard May 22, 1928 Payne Sept. 15, 1931 Bing Oct. 6, 1931 Bower et a1 Oct. 30, 1934 Stearns et a1 Feb. 16, 1937 Kiekhaefer July 30, 1940 Griffin Sept. 17, 1940 Gilfillan May 15, 1945 Sampietro et a1. July 4, 1950 FOREIGN PATENTS Country Date Great Britain Aug. 17, 1917 Great Britain Oct. 8, 1948 France July 23, 1948 

